The arithmetic sequence $(a_i)$ is defined by the formula: $a_1 = -9$ $a_i = a_{i-1} + 3$ What is $a_{8}$, the eighth term in the sequence?
Explanation: From the given formula, we can see that the first term of the sequence is $-9$ and the common difference is $3$ To find the eighth term, we can rewrite the given recurrence as an explicit formula. The general form for an arithmetic sequence is $a_i = a_1 + d(i - 1)$ . In this case, we have $a_i = -9 + 3(i - 1)$ To find $a_{8}$ , we can simply substitute $i = 8$ into the our formula. Therefore, the eighth term is equal to $a_{8} = -9 + 3 (8 - 1) = 12$.